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Actin microfilaments in paramecium: localization and role in intracellular movements
Abstract:
Using heavy meromyosin (HMM) or the fragment S1 of myosin as probes for actin microfilaments, we studied their organization in Paramecium both by fluorescence and electron microscopy. In interphasic cells, HMM decorates (a) most prominently the periphery of nascent and young food vacuoles and their route during the early phase of their intracellular transit; (b) a thin meshwork radiating from the gullet throughout the cytoplasm; (c) a small area beneath the pore of contractile vacuoles and beneath the cytoproct when open to release food residues. Most of these HMM-decorated structures are in close contact with microtubular arrays. All HMM decoration disappears in dividing cells and in cytochalasin-treated cells. In vivo, the drug immediately blocks food vacuole formation but does not affect cytokinesis, cyclosis, contractile vacuole pulsation, defecation, or nuclear movements. The data show that, as in the cells of other organisms, actin microfilaments form defined arrays that undergo physiologically controlled cycles of assembly/disassembly. These arrays contribute (at least in the phagocytotic process) to diverse types of movement: constriction, membrane fusion, and migration of food vacuoles. However, aside from their massive concentration along the phagocytotic tractus, actin microfilaments are neither major structural components of Paramecium cytoplasm nor the only cytoskeletal components ensuring motility or contractility processes.
Insights
Actin microfilaments (HMM) in Paramecium are crucial for food vacuole formation and transport. These dynamic structures assemble and disassemble, aiding in cellular movements like phagocytosis, but are not the sole cytoskeletal components involved.
Area of Science:
- Cell Biology
- Cytoskeleton Dynamics
Background:
- Actin microfilaments are essential cytoskeletal components involved in various cellular processes.
- Understanding their organization and function in unicellular organisms like Paramecium provides insights into fundamental cell biology.
Purpose of the Study:
- To investigate the organization and dynamic behavior of actin microfilaments in Paramecium.
- To determine the role of actin microfilaments in cellular motility, particularly in phagocytosis and food vacuole transport.
Main Methods:
- Utilized heavy meromyosin (HMM) or S1 fragment of myosin as probes for actin microfilaments.
- Employed both fluorescence and electron microscopy for visualization.
- Observed the effects of cytochalasin treatment on cellular processes.
Main Results:
- Actin microfilaments form distinct arrays in interphasic Paramecium, decorating food vacuoles, a cytoplasmic meshwork, and areas near contractile vacuoles and the cytoproct.
- These actin structures are often associated with microtubular arrays.
- Actin decoration disappears in dividing and cytochalasin-treated cells; cytochalasin immediately inhibits food vacuole formation but not other observed processes.
- Actin microfilaments exhibit cycles of assembly and disassembly, contributing to food vacuole formation, membrane fusion, and migration.
Conclusions:
- Actin microfilaments in Paramecium are dynamically regulated, with defined arrays involved in phagocytosis and food vacuole trafficking.
- While crucial for phagocytosis, actin microfilaments are not the primary structural components of the cytoplasm nor the sole mediators of motility and contractility in Paramecium.
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